The present invention relates generally to the fabrication of composite structures and, more specifically, to a delivery head apparatus for placement of fiber elements employed in fabrication of such structures. The delivery head apparatus of the present invention is particularly suited for placement of discrete, elongated fiber elements in mutually superimposed relationship to define reinforcement or stiffening members for the interiors or exteriors of composite shells.
Composite reinforced, or "grid-stiffened", shell structures, such as shrouds, casings, fuel tanks, airfoils or wing skins, and fuselage panels would provide recognized advantages in aerospace applications over conventional metal (typically aluminum) assemblies in terms of relatively lower weight and higher strength for the composite structures. However, acceptance of all-composite structures has been hampered by the lack of a demonstrated, repeatable and inexpensive fabrication methodology and apparatus for effecting same in an automated manner. Thus, many such structures are limited to high-performance, military aerospace applications, as not being cost-competitive for commercial aerospace uses. For the same reason, present non-aerospace usage of composite structures is somewhat limited, as in many cases, metal structures accommodate design requirements at a lesser cost.
A composite shell structure may require discrete internal or external reinforcing elements or stiffeners, hereinafter referred to generally as "ribs", adjacent a continuous shell structure, to provide enhanced stiffness to the shell structure in terms of torsional and bending resistance. The larger the composite shell structure and the greater the stresses to which it may be subjected in operation, the greater the need for such reinforcement. Reinforcing elements may be discrete and remote from each other, may be disposed in mutually parallel relationship, or may be disposed in intersecting relationship to define a grid-type arrangement, wherein a large plurality of rib elements placed at intersecting orientations defines an array of triangular, square, otherwise polygonal or otherwise-shaped cavities. The composite shell structure may then be formed over the grid, or formed first on tooling, and then the grid formed onto the shell. One specifically favored grid pattern due to its non-preferential stress accommodation capabilities is a so-called "isogrid", wherein each cavity is triangular with three 60.degree. apices. Reinforcing elements have conventionally been fabricated from metal elements (as in prior art all-metal assemblies) which, in the case of composite structures, is disadvantageous in terms of weight as well as accommodating differences in the coefficient of thermal expansion (CTE) of the metal reinforcing elements and that of the adjacent fiber composite shell in the final structure.
It is known in the art to hand-place resin (epoxy) impregnated fiber element "tows" onto tooling to create a grid-type arrangement of rib-like stiffeners. As noted above, the stiffeners may be formed directly on the tooling, or on a shell preform of composite fibers which has itself been applied to the tooling. A number of tows are typically laid up on a mandrel or other tooling in vertically superimposed, or stacked, relationship to define each stiffening rib of a desired height and cross-section. The tows are then cured simultaneously under applied heat and pressure with a contiguous composite shell (laid up either before or after the ribs on the tooling), as known in the art, to define the overall reinforced composite structure. However, hand-placement is slow, fails to provide an acceptable product in terms of quality due to the difficulty in applying the tows in exact superimposition and with precise, controlled pressure, and thus renders the fabrication process too labor-intensive and thus too expensive for relatively high production volumes such as are required for commercial applications. Moreover, hand layup techniques become ever-less satisfactory with both increasing complexity of a stiffening grid pattern and an increasing number of tow layers.
It is also known in the art to fabricate the stiffeners by automated application, or "winding", of fiber elements in the form of continuous filaments onto a cylindrical mandrel. However, filament winding has exhibited perceptible deficiencies in terms of inaccuracy of fiber placement, as well as insufficient compaction of the fiber. Further, filament winding generates an excess of fiber scrap since it requires a continuous, turnaround path when each end of a mandrel is reached; the filament turnarounds at the ends of the mandrel do not form part of the final structure, and so are cut off and discarded. In addition, known filament winding techniques provide no capability to "steer" the fiber filament to accommodate desired variations from a preprogrammed path, to place fiber on complex geometry mandrels, including those exhibiting concave exterior portions, or to terminate fiber element placement at a target point on tooling and then restart application of a new fiber element at a new target point. Filament winding demonstrates particularly severe limitations where stiffening members cross or intersect, due to any inability to eliminate or reduce fiber element buildup at the nodes where fiber elements oriented in two or more directions cross. Finally, known filament winding techniques lack the capability to place fiber at a zero degree angle, i.e., parallel, to the longitudinal axis of rotation of the mandrel.
Thus, there is a need in the art for an apparatus capable of placing discrete fiber elements in desired lengths and at desired angles along specified paths onto tooling so as to form stiffening structures onto which a blanket of composite fibers may be laid up to result in a reinforced, all-composite shell structure.